Hydraulic drillstring sound generator

ABSTRACT

Acoustic radiators for coal bed methane or shale gas production are configured to be strategically placed on drillstrings within gas production exhaust boreholes and hydraulically powered so they will radiate intense harmonic sonic waves to shake the solid media immediate to the wall areas. The gas volume output that can be realized by an exhaust well depends highly on the permeability of the media, especially at inside faces of the borehole. The shaking half-opens up fractures and pores in the solid media. Thus the permeability of the media to gas improves under such shaking and gas collection efficiencies are improved. The beneficial effects can be increased by locating two or more acoustic radiators proximate to one another in a phased relationship.

RELATED APPLICATIONS

This Application claims benefit of U.S. Provisional Patent Application Ser. No. 61/675,855, filed Jul. 26, 2012, by Dmitry A. Kasyanov and Victor Zhoglikov, and titled, HYDRAULIC DRILLSTRING SOUND GENERATOR.

BACKGROUND

1. Field of the Invention

The present invention relates to devices and methods for generating sound vibrations, and more particularly to intrinsically safe drillstring vibrators suitable for use in horizontal and inclined wellbores that cannot be assumed to be completely inundated with liquids.

2. Description of the Prior Art

Methane (CH₄), firedamp (CH₄+various hydrocarbon gases), and natural gas (mostly CH₄ and ethane C₂H₆), are customary constituents of every coalbed deposit, and are formed in situ by Nature when the coalbed takes shape. Such gases are adsorbed by the coal, e.g., they occupy the interior of surface areas over the entire parent coal matrix. The adsorption surface area of coal can be very large, e.g., about one billion square feet per ton of coal. Coalbeds can store significantly more gas than a typical and otherwise similarly sized natural gas deposit.

Coal mining is dangerous because every coal deposit includes some amount of explosive and flammable methane and other gases. As a general rule, the amount of methane adsorbed by the coal itself is proportional to the grade of the coal. The higher the grade of coal, the higher will be the methane content. Also, the deeper the coalbed lies, the higher will be its gas content because the pressure in a coalbed is always proportional to its depth. The degree of gas sorption increases with pressure. The reduction in pressure needed to get the gas to desorb and move out to the exhaust well collectors can be predicted with a desorption isotherm for various given temperatures.

A typical complication is coal beds are very often inundated with ground water. Hydrostatic pressures caused by such water will increase the pore pressure in a coalbed and its gas sorption. Desorption isotherms can be used to predict the hydrostatic pressure drop needed to recover methane from coal.

In the past, good collection methods and equipment needed to harvest methane from coalbeds simply did not exist. So the methane represented a nuisance and not a potential way to make a profit. Methane poisons the air miners needed to breath, and worse, it's explosive. Even today, when modern methods and equipment can be employed to great success, serious and frequent mining explosions and disasters continue to occur that could have been avoided if the methane or firedamp had simply been removed before coal mining operations began. These accidents have been especially common recently in the coal mines of Russia, Ukraine, China and the Upper Big Branch (UBB) mine in the United States. But no coal mine in the world is immune.

When removed and reinjected with water or carbon dioxide, the explosive potential can be significantly reduced. Reinjection should be made an integral part of mining best practice when mining through faults, dykes and paleochannels, and other coal seam anomalies.

Methane production ahead of mining has become a widespread way to protect against methane-related accidents and to increase profits, e.g., by selling off the collected methane. The harvesting of methane can add so much to the profits, even coalbeds or strata too deep or too poor to support profitable coal production is becoming an attractive way to convert inchoate hydrocarbon reserves into real revenues.

Coal mine gas production holes were once simply used to help ventilate mines and to minimize the coal-production risk due to mine gases. Now, coal mine operations recognize that profits can be made by gas production and sales. Simply releasing the gas into the atmosphere wastes resources and money, and the gas can easily pollute the environment. Recent experience in mine degasification has led to the development of gas production independent of coal mine operations.

Vertical and horizontal boreholes drilled into coalbed and shale gas reservoirs are widely used methods for gas production. Coalbed deposits are best degasified for safety before starting coal mine production. Sometimes vertical or directionally drilled boreholes are drilled as gassers independent of any intent to later mine any coal involved.

A second, more benign gas can be injected in a deposit to push out the methane, firedamp, and natural gas. Prior art methods use water, nitrogen, carbon dioxide, and vitiated air injections into coalbeds to force out the resource gases. A system of injection and collector holes is needed for this.

The actual gas content, the pressure in the coalbed, the presence of water, and the permeability all affect how much gas can be recovered and at what cost. A fracturing pattern inside a coalbed, called “cleavage,” is one factor that determines the in-place permeability. Cleavage and stratification can ease the flow of gases and fluids inside a coal bed.

For example, a coal bed with a low gas content and a high hydrostatic pressure on the desorption isotherm requires extra production of water for every unit of produced methane. Similarly, gas recovery from a coalbed with a very low permeability requires intense fracturing. In many cases, efficient gas recovery is not possible because appropriate production-enhancement technologies do not exist.

Drilling-in boreholes in coal beds causes localized pressure reliefs and creates pressure gradients as the methane flows to the borehole output wells. A diffusion flux can be generated throughout the coal matrix with laminar flows through the coal bed fractures around the boreholes. Any ground water present must be pumped out to reduce the coalbed pressures enough so the gases desorb from the coal. The faster the water is removed, the faster the retained gas will be released.

The volume of gas output that can be realized at any exhaust well depends on the permeability or permeability of the walls and faces of the borehole. These behave like filter matrices, with the most restrictive parts in the collector zone being not more than a few diameters from the center of the exhaust well.

Embodiments of the present invention are therefore directed to enhancing the permeability of the material surrounding the exhaust wells. The more permeable that such immediate area around the exhaust borehole can be made, the higher will be the volume of gas produced, all else being equal.

The formula for a pressure gradient distribution in a one-dimensional radial flow from a circular supply circuit with radius R_(c), and pressure P_(c) to a concentric borehole with effective radius r_(b), and face pressure P_(b), is as follows:

${{P(r)} - P_{c}} = {\frac{P_{b} - P_{c}}{\ln \left( \frac{R_{c}}{r_{b}} \right)}{{\ln \left( \frac{R_{c}}{r} \right)}.}}$

Such describes a logarithmic pressure distribution between the supply circuit and the borehole at the center. A majority of the pressure differentials are concentrated in the walls immediately surrounding the boreholes. For example, given R_(c)≈100 meters, and r_(b)≈0.1 meter, more than one-third of the pressure difference is dropped across the last one meter to the borehole core. Over one-half is dropped across a zone of radius≈3 meters. The situation is even more pronounced for boreholes with smaller radii r_(b).

Drilling and production can generate particles of mud filtrate and small coal that can form a filter cake that will reduce or completely shut-down an exhaust well bore. The borehole output for the same face pressure can be considerably reduced by critical-zone pore-clogging, or colmatation. For example, it is estimated a tenfold decrease in permeability in an area of radius 0.5 meter for r_(b)≈0.1 meter results in a threefold decrease in the output. If the same decrease in permeability takes place in an only slightly larger 0.2 meter radius zone, then the output is reduced by much less than before, e.g., 40%.

A principal benefit of acoustically vibrating the inside faces of the boreholes in porous and fractured media is to increase its permeability. The more permeable that such immediate area around the exhaust borehole can be made, the higher will be the volume of gas produced, all else being equal.

Vibration and acoustic effects can be used to intensify many mass-transfer processes. But all too often, no elastic-vibration devices are available that are suitable for use in the harsh environments in which they must operate. Such environments include the drilling of extended horizontal wells for preliminary methane drainage from coal beds, and the production of coal methane and shale gas.

The intensifying action of elastic oscillations can be used to commercial advantage in at least two ways. The mass-exchange processes in the spaces surrounding the well are improved, e.g., as in methane output intensification of gas collection wells drilled in coal seams. The boring resistance and friction to the drill bits used during drilling can also be reduced if intense elastic oscillations are employed in drilling horizontal and/or inclined wells.

The medium inside extended horizontal gas production wells is typically characterized by insignificant acoustic impedance. The insides of the wells and their walls are not usually completely flooded. Such wells are often filled with a mix of gases and liquids, where the gases predominate. The acoustic impedances in the gaps can therefore be small, and will inhibit making a good acoustic contact between the system radiators, the downhole medium, and the wellbore walls.

The gas production benefits of exposing coal or shale layers to intense acoustic vibrations can be further improved by maximizing the run of the wellbore wall that actually receives the vibrations. The linear drillstring length of each of the acoustic vibrators put down a gas well should be as long as is practical.

Even before a gas field is placed into production, intense acoustic vibrations from vibrators placed just behind the drill bits can be used to speed up the drilling and reduce the overall costs. Intense acoustic vibrations applied to drill-bits can reduce friction and wear. The vibrators must be able to move freely with the drillstring, and should be powered the same way as the downhole motor, e.g., liquid under pressure.

Horizontal and inclined wells can be assumed to benefit from good acoustic contact inside because the acoustic vibrators will lie down and touch the wellbore wall under their own weight.

The technical solutions for vibrators that have been developed are as varied as the technical fields in which they have been applied. Many conventional devices look like they could be used for partially flooded or even dry horizontal and inclined wells. However, on closer inspection it can be seen that conventional downhole elastic oscillation sources are only suited for stationary, small-scale operations in liquid-filled wells.

In the construction industry, mechanical vibrators on the ends of thick hoses or on the outsides of form walls are widely used to amalgamate and strengthen concrete pours. The shaking liquefies the loose concrete pour within a circular field of action before it cures so the concrete will flow into every void and any trapped air will bubble out.

Many such conventional vibrators look like they would suit the geometries found inside horizontal and inclined well. So they are often used as starting basis in vibrator designs requiring a cylindrical body with a relatively small radius. In a majority of designs, the vibrations are directly generated by rotating an eccentric mass. Various kinds of motors are employed to drive the rotations. For example, U.S. Pat. No. 2,597,505, U.S. Pat. No. 3,549,130, U.S. Pat. No. 5,564,824, and U.S. Pat. No. 6,811,297, disclose external electric drive; U.S. Pat. No. 2,891,775, U.S. Pat. No. 3,162,426, U.S. Pat. No. 3,171,634, U.S. Pat. No. 3,193,256, U.S. Pat. No. 3,365,965, U.S. Pat. No. 3,376,021, U.S. Pat. No. 4,293,231, U.S. Pat. No. 4,300,843, and U.S. Pat. No. 4,428,678, describe compressed air being used to drive a planetary or free rotor along the internal surfaces of a stator; and, U.S. Pat. No. 2,960,314, U.S. Pat. No. 3,229,961, U.S. Pat. No. 3,290,952, U.S. Pat. No. 3,318,163, U.S. Pat. No. 3,357,267, and U.S. Pat. No. 4,682,896, direct pressurized fluids to create a vortex flow inside a working volume to entrain a rotor.

The advantages of using electric drive include being able to control the oscillation frequencies over a wide range. Electric drives are difficult to make safe in explosive environments, like in gas wells and coal mines. Pneumatic drive vibrators are similarly difficult to make safe, there is a high probability of forming and igniting an explosive mix of air and flammable gas.

Conventional vibrators that direct tangential inputs of compressed fluid into the working volumes have problems with the longitudinal (axial) stabilization of their rotors. A lack of stabilization leads to energy losses due to rotor friction and uncontrolled collisions against inside surfaces. Destabilized rotors can generate intense random noise components. Destabilization can result when significant pressure fluctuations occur in the input flow.

U.S. Pat. No. 4,682,896 by Halilovic describes an attempt to solve this problem. Unfortunately, the vibrator disclosed will not work in the horizontal or even inclined positions. The rotor is weighed to be upright in gravity. The liquid flow is fed from the bottom and lifts the rotor, starting it to spin. The flow is taken out through vertical channels. If the rotor gets tilted, the longitudinal axis will be angled with respect to gravity. The liquid flow can then only act on one end of the rotor, making planetary movement of rotor around the axis of stator impossible. When tilted, the vibrator tends to generate wideband noise, and is why the most important thing in such type of vibrators is the stabilization of the rotor movement around stator.

There are some other technical solutions that were based on a fluid driven screw type motor, e.g., as originally described by Moyneau, 1939, and employed by Bodine in U.S. Pat. No. 4,824,258 and Kochnev in Russian Patent RU2162509. See, French Pat. No. 850,942 to Moyneau S. A. R. L. issued on Sep. 25, 1939, and Page 155 of Pumps by Kristal and Annett, McGraw-Hill Book Company, 1940. In principle, vibrators of this type for use in horizontal or inclined wells had no market. Their uses are limited by the costs of the expensive rotor and stator units they require, and the very short maintenance overhaul periods in the hundreds of hours. Well drilling screw downhole motors are economically sound, but using such motors in vibrators does not seem to be worthwhile.

What is needed are vibrators suitable for use in horizontal and inclined wells. The vibrations should be driven by pressurized liquid. The vibrator design must all free travel all along horizontal or inclined wells. The vibrator design should permit serial connections of several vibrators, e.g., inline and powered from a single pressurized-liquid source.

SUMMARY OF THE INVENTION

Briefly, acoustic radiator embodiments of the present invention for coal bed methane or shale gas production are configured to be strategically placed within gas production exhaust boreholes and hydraulically powered so they will emit intense harmonic sonic waves to shake the solid media immediate to the wall areas. The gas volume output that can be realized by an exhaust well depends highly on the permeability of the media, especially at inside faces of the borehole. The shaking half-opens up fractures and pores in the solid media. Thus the permeability of the media to gas improves under such shaking and gas collection efficiencies are optimized. The beneficial operational effects can be increased by locating two or more acoustic radiators in a linear series proximate to one another in a phased relationship.

In another aspect of the present invention, acoustic radiators are placed along drill string or just behind a drillstring drill bit to reduce friction and wear.

An advantage of the present invention is inert gases and liquids can more easily and effectively be re-injected into coal seams and anomalies to significantly improve miner health, safety, and productivity.

These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is a cutaway perspective diagram of an underground coal deposit that is being drained of its methane, firedamp, and/or natural gas with acoustic radiator embodiments of the present invention that stimulate improved permeability of the media immediately around the exhaust well boreholes;

FIG. 2 is a perspective view diagram of a planetary bushing orbiting on a shaft under the influence of spiraling flows in a general embodiment of the present invention that can produce strong audio range vibrations deep underground;

FIGS. 3A and 3B are cross sectional cutaway and exploded assembly view diagrams of a drillstring vibrator embodiment of the present invention suitable in the application diagramed in FIG. 1; and

FIGS. 4A and 4B represent a shaft and a free bushing in an embodiment of the present invention that are used together to control frequency instabilities that can be caused by operating-pressure fluctuations in the hydraulic inlet feed lines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 represents a coal bed deposit mining operation for methane, firedamp, and/or natural gas, and is referred to herein by the general reference numeral 100. A pair of exploratory vertical boreholes 102 and 104 have been drilled from the ground surface to allow for electronic sensors that can imagine and characterize a coalbed 106. A pair of directional drillstrings 108 and 110 have been used to first bore vertically to the right depth and then horizontally into the coalbed 106. A paleochannel 112 comprising sandstone represents a typical flaw or anomaly in the coal bed 106.

The coal bed 106 has naturally occurring adsorbed methane, firedamp, and/or natural gas. It may also be inundated with groundwater. The depth of the deposit and any groundwater will pressurize the gas adsorbed by the coal. Drillstrings 108 and 110 can be used to remove the groundwater and vent the gas. Such will promote desorption and the drillstrings 108 and 110 and their boreholes are used exhaust the natural gas.

Drillstrings 108 and 110 are fitted with acoustic radiators 114-121 and above-ground, high power hydraulic or inert gas pressurizing pumps (not shown). Inert gas or hydraulic pressure flows are sent down drillstrings 108 and 110. The acoustic radiators 114-121 are each independently configured to convert the flows to strong acoustic vibrations. The acoustic radiators 114-121 produce oscillation modes selected to have maximum effect in gas production. Different modes can be used simultaneously, and phasing between the radiators will also have benefits.

Such sound vibrations shake the coal media and increase gas permeability especially near the boreholes. Increased desorption gas flows result that can be exhausted and sold.

Phased arrays can be used to focus or concentrate the sound energy. In such case, the radiators are placed within range of each other. Otherwise, they are spaced far apart to lengthen their zone of effect along the drillstring.

Embodiments of the present invention are useful to degasify coal beds with borehole acoustic equipment, and inert gases and liquids can more easily and effectively be re-injected into coal seams and anomalies to enhance miner health, safety, and productivity. In particular, subjecting the near-hole area to strong sound waves improves the permeability of the media to natural gas. These further include equipment for injecting a second gas into coalbed in order to drive out the desorbing methane.

The choice of what kind of acoustic radiators 114-121 to use and how to match the radiators with the surrounding media are practical challenges that are overcome by the present invention. Electrically operated radiators are dangerous because they can spark an explosion of the very gas being extracted. Connecting them and fitting them with an adequate power source is also problematic. Not placing the radiators in direct contact with the solid inside faces of the boreholes can result in poor acoustic impedance matching, and all the benefits can be lost because strong enough vibrations do not reach the media.

Multiple acoustic radiators 114-117, for example, can be mounted on pipe drillstring 108 at critical points and with critical frequency outputs compared to each other so as to produce a phasing of outputs extend or intensify the media zone in which the permeability is increased so the gases or liquids can be removed.

Mechanical sound radiators not powered by electricity are attractive in this application. Herein are described mechanical sound radiator embodiments of the present invention that will be suitable for the applications described in relation to FIG. 1. The majority of the prior art downhole sources of elastic oscillations are not suitable for use here, since they are mainly designed for stationary small-scale operation in liquid-filled wells.

Drillstring vibrator embodiments of the present invention generally include a cylindrical external housing that receives a pressurized hydraulic or inert gas flow at one end and passes the flow to the opposite end. A coaxial shaft is fitted with bushing that can eccentrically roll around in constant contact with the outer surface of coaxial shaft in a working space bordered by the external cylindrical housing. The pressurized flows are channeled and ported to cause the bushing to oscillate. This structure allows the vibrators to be organized into linear chains.

Experiments with various embodiments of vibrators show the basic operating frequency solely depends on the tangential velocity of liquid in the working area, e.g., the pressure difference and geometry of its internal space. One prototype with an external diameter 80-mm, a working-space diameter 50 mm, and a vibrator pressure difference ten bar generated oscillations with a frequency of 110-Hz.

The vibrator oscillation frequency is almost independent of the free-bush mass and geometry. The intensity of the vibrations generated depends on the kinetic energy developed by the planetary bushing during rotation. Whereas the dependence on the free-bushing mass is linear, the eccentricity dependence is quadratic. For example:

${{m\frac{V^{2}}{2}} = {m\frac{\left( {2\pi \; f\; ɛ} \right)^{2}}{2}}},$

where m the free-bush mass, V is the linear velocity of the bush center of mass, f is the free-bush rotating frequency, and ε is the eccentricity of the free-bush center of mass with respect to the vibrator symmetry axis. The aforementioned vibrator model with the free-bush weight 450-grams and eccentricity about five millimeters and the same pressure difference developed an acceleration of forty-four m/s², which is about 4.5-g.

FIG. 2 represents a basic acoustic vibrator 200 in an embodiment of the present invention. A planetary bushing 202 is loosely mounted on a hollow shaft 204 and is induced to eccentrically rotate in orbit on the shaft by two spiral flows 206 and 208. A hydraulic or inert gas inlet 210 can be used. Part of inlet 210 is directed down the hollow inside of shaft 204 to become spiral flow 208. Exhaust ports 212 and 214 provide a means for the flows to escape a working space once they have done their jobs in a cylinder (not shown) and working space that surrounds the planetary bushing 202. Flows from the exhaust ports 212 and 214 are combined into an outlet 216.

The basic mechanism represented in FIG. 2 can be implemented in a number of various ways. FIGS. 3A and 3B represent one possible way an acoustic vibrator could be implemented and succeed in the application shown in FIG. 1.

FIGS. 3A and 3B represent a drillstring vibrator 300 with a hollow, a cylindrical housing 301 with drain channels 302, a hollow shaft 303 with a central bore 304, a first spiral a 305, a second spiral nozzle 306, and a free eccentrically rolling planetary bushing 307. Bushing 307 floats loosely around on shaft 303 and orbits inside a cylindrical working space 308.

High pressure liquid, for example, is spirally introduced to working space 308 by peripheral grooves 309 and 310, respectively disposed in the first and second spiral nozzles 305 and 306. The flow to channel 309 comes directly from a feed line inlet 311. Channel 310 is connected with inlet 311 by the hollow central bore 304 in shaft 303. Exhaust flows exit working space 308 through two opposite drain channels 302 and out to drain 313.

In operation, a pressurized liquid flow F from inlet 311 is divided into two parts, F₁ and F₂. Flow F₁ proceeds directly to working space 308 by peripheral groove 309 of spiral nozzle 305. Flow F₂ passes through bore 304 into a chamber 312 and is turned around as flow F₃. It then is spun into working space 308 by peripheral groove 310 on spiral nozzle 306. Flows F₁ and F₃ enter working space 308 in balance. The longitudinal components of flows F₁ and F₃ are in the same axis and counter-propagating.

Bore 304 should not be so small in diameter as to impose a significant restriction to flow F₂. The cross-sectional area of bore 304 should be more than the total cross-sectional areas of peripheral grooves 309 and 310.

Flows F₁ and F₃ will vortex when they enter working space 308, and their tangential components should act equally on both ends of rolling planetary bushing 307. The vortex flows swirling around rolling planetary bushing 307 pull it into a fast orbit by viscous friction. Centrifugal forces will cause the internal bore of bushing 307 to press hard on the external surfaces of shaft 303 when orbiting. Planetary bushing 307 will accelerate in its orbit velocity to match the velocities of the swirling flows. The planetary motion generates strong audio range vibrations that are efficiently coupled outward by shaft 303, spiral nozzles 305 and 306, a pair of shaft supports 320 and 321, and ultimately to housing 301. Shaft supports 320 and 321 are each respectively ported to allow flows F₁ and F₃, and they are pinned to spiral nozzles 305 and 306 to maintain their orientation.

Rolling planetary bushing 307 is longitudinally stabilized by the counter-propagating longitudinal components of flows F₁ and F₃ at both its ends. After doing its job, the flows drain out as flow F₄ through drain 313 with flow F₅ via drain channel 302 of housing 301. To reduce hydrodynamic resistances, the cross-sectional area of the drain channel 302 and the area of the hole connecting working space 308 and drain channel 302 significantly exceed the total cross-sectional area of all peripheral grooves which bring pressurized liquid to working space 308 of vibrator 300.

The spiral channels 309 and 310 are configured to introduce a flow with a spin into working space 308 that will ensure reliable acoustic contact of bushing 307. The pitch of the spiral channels is selected to find a good balance between the tangential and longitudinal components of flows F₁ and F₃. Experiments show that best results will be obtained when the pitch is in the range of 5-10°. All sharp edges should be rounded off to avoid cavitations that would otherwise occur.

Experiments have shown that the optimal length of planetary bushing 307 is approximately 10% less than the inside length of working space 308. During operation, planetary bushing 307 rolls around on shaft 303.

When hydrodynamic vibrators are driven by high-power pumps, the pressure in the feed lines can fluctuate significantly. Vibrator frequency stability can be improved by using a shaped shaft, for example, in the form of a cone of revolution with an apex a little less than 180°. Model experiments showed that complete stabilization can be realized at an angle of 177°. In this case, the internal surface of the planetary bushing 307 rolls around on the outside surface of shaft 303.

The oscillation energy, and output amplitude, can be increased by making unbalancing the planetary bushing. The oscillation energy is directly proportional to the mass planetary bushing 307, but its dependence on the eccentricity is quadratic.

Prototype vibrators were made with an external diameter of eighty millimeters (3.15″), the inside nominal diameter of working space 308 was fifty millimeters (1.97″), and the planetary bushing 307 had eccentricity about five millimeters (0.2″) and weight about 450 grams (almost a pound). The frequency of the vibrator-generated oscillations depends on the pressure difference on the vibrator. E.g., 50 Hz at two Bar rising to 110-Hz at ten Bar. The vibrators produced intense and harmonic sinusoidal oscillations.

The longitudinal working position of free bushing 307 on shaft 303 can be destabilized by pressure fluctuations in the hydraulic feed lines connected to inlet 311, especially if these fluctuations become significant. Such destabilizations can cause the operational frequency to wander, efficiencies to drop, and can be deleterious. The solution is to impart a slight longitudinal curve to the mating surfaces of free bushing 307 on shaft 303.

FIGS. 4A and 4B represent a shaft 402 and a free bushing 404 in an embodiment of the present invention that are used together to control frequency instabilities caused by operating-pressure fluctuations in the high-pressure hydraulic feed lines. The outside surface of shaft 402 has two slightly tapered cone sections 406 and 408 on either side of a middle section 410. The tapered cone sections 406 and 408 narrow toward respective outer shoulders 410 and 412. The effect is to make shaft 402 slightly thicker in a middle section 410 and slightly narrower at its extremities, e.g., an angle-A will optimally be about 177°. Support ends 416 and 418 are right cylinders and the whole of shaft 402 has an end-to-end hollow 420. Overall, shaft 402 approximately expresses a convex axi-symmetrical surface.

The inside surface of free bushing 404 is slightly concave and fabricated to match and mate with the slightly convex external surface of shaft 402. A central groove 422 is flanked by hollow cone sections 424 and 426 that taper down slightly to end sections 428 and 430. The tapers taken together are shown here as an angle-B, about 177°.

Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention. 

What is claimed is:
 1. An acoustic radiator, comprising: a rotating assembly configured to be powered into spinning by a hydraulic or inert gas flow from a feed line, and to produce a harmonic acoustic field when rotating under power; a cylindrical housing for enclosing the rotating assembly and hydraulic or inert gas flow, and for mechanically coupling said harmonic acoustic field to its outside surfaces; wherein, the rotating assembly and cylindrical housing are configured for attachment to a drillstring to receive said hydraulic or inert gas flow, and further configured for operation inside a horizontal or inclined borehole; and wherein, said outside surfaces of the cylindrical housing are configured to contact and induce said harmonic acoustic field into the surrounding media of said borehole.
 2. The acoustic radiator of claim 1, wherein: provides for improvements in coal bed methane, firedamp, or natural gas production by increases in the permeability of porous and fractured underground media nearby such that gases and/or liquids can be more easily removed.
 3. The acoustic radiator of claim 1, wherein: provides for improvements in miner health, safety, and production by increases in the permeability of porous and fractured underground media nearby such that inert gases and/or liquids can be more easily re-injected.
 4. The acoustic radiator of claim 1, wherein significant variations in the pressure delivered by the feed line that would otherwise produce vibrator frequency instability are controlled by including a shaped shaft in the form of a cone of revolution with an apex of just less than 180°, and such that the internal surface of a planetary bushing follows the internal surface of the shaped shaft.
 5. The acoustic radiator of claim 4, wherein complete vibrator frequency stabilization is realized when the shaped shaft in the form of a cone of revolution has its apex set to an angle of 177°.
 6. The acoustic radiator of claim 1, wherein natural gas production of an exhaust well can be improved by increasing the permeability of the surrounding media adjacent to it.
 7. A system for increasing the permeability of porous and fractured media, comprising: a pipe for entering a vent in a porous or fractured media to remove gases or liquids; a first acoustic radiator mounted on said pipe; a hydraulic or inert gas flow forced down the pipe to cause the acoustic radiator to vibrate; wherein, a coupling of vibrations into said media causes its permeability to be increased so said gases or liquids can be more readily removed from said media; wherein, the first acoustic radiator comprises: a rotating assembly configured to be powered into spinning by a hydraulic or inert gas flow from a feed line, and to produce a harmonic acoustic field when rotating under power; a cylindrical housing for enclosing the rotating assembly and hydraulic or inert gas flow, and for mechanically coupling said harmonic acoustic field to its outside surfaces; wherein significant variations in the pressure delivered by the feed line that would otherwise produce vibrator frequency instability are mitigated by including a shaped shaft in the form of a cone of revolution with an apex of 177°, and such that the internal surface of a planetary bushing follows the internal surface of the shaped shaft.
 8. The system of claim 7, further comprising: a second acoustic radiator similar to the first and mounted on said pipe to extend the media zone in which the permeability is increased so said gases or liquids can be removed.
 9. The system of claim 7, further comprising: a second acoustic radiator similar to the first and mounted on said pipe at a critical point and with a critical frequency output compared to the first acoustic radiator that produce a phasing of outputs extend or intensify the media zone in which the permeability is increased so said gases or liquids can be removed.
 10. A method for stabilizing the operational frequencies of a hydraulically powered down-hole acoustic radiator when subjected to significant variations in feed line pressures, comprising: configuring a rotating assembly to be powered into spinning by a hydraulic or inert gas flow from a feed line, and to produce a harmonic acoustic field when rotating under power; enclosing the rotating assembly and hydraulic or inert gas flow in a cylindrical housing, and mechanically coupling said harmonic acoustic field to its outside surfaces; configuring the rotating assembly and cylindrical housing for attachment to a drillstring to receive said hydraulic or inert gas flow, and further configuring such for operation inside a borehole; and controlling vibrator frequency instabilities by including a shaped shaft in the form of a cone of revolution with an apex of just less than 180°, and such that the internal surface of a planetary bushing follows the internal surface of the shaped shaft.
 11. The method of claim 10, wherein the shaped shaft in the form of a cone of revolution has an apex with an approximate angle of 177°.
 12. (canceled) 